(1) Field of the Invention
This invention relates to a secondary air supply system for an internal combustion engine which supplies secondary air, including some of intake air, which flows in an induction system of the internal combustion engine, and atmospheric air, to an exhaust system of the internal combustion engine.
(2) Background Art
Some conventional internal combustion engines are configured so as to supply secondary air to an exhaust system of a combustion engine in order to improve exhaust emission. In such an internal combustion engine, secondary air is supplied to the internal combustion engine through a supply path of the secondary air during a period wherein a catalyst, provided in the exhaust system, is not yet activated, such as when the internal engine is started while it is still cold. When the secondary air is supplied to the exhaust system, unburned gas contained in exhaust gas inside of the exhaust system is burned, and hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas become oxidized.
By oxidizing hydrocarbon and carbon monoxide in the exhaust gas, the residual amount of hydrocarbon and carbon monoxide is reduced, and hydrocarbon and carbon monoxide are inhibited from being externally released while a catalyst is not activated. Therefore, exhaust emission is improved. Moreover, since unburned gas is burned, the temperature of the catalyst quickly increases. Therefore, the catalyst comes into an activated state quickly after the cold internal combustion engine is started.
For the above-described secondary air supply system of an internal combustion engine, Unexamined Japanese Utility Model Publication No. 5-10735 discloses a system wherein a delivery pipe, disposed in the downstream of a supply path for secondary air, is connected to an exhaust manifold via two communication pipes. The exhaust manifold includes one pair of flanges and bosses respectively formed in an integrated manner. The flange is connected to a cylinder head of the internal combustion engine. The bosses are provided so as to connect the communication pipes in the vicinity of the connecting portion of branching pipes in which exhaust gas flows from respective combustion chambers. Inside of the delivery pipe and inside the exhaust manifold are communicated through the communication pipes.
On the other hand, Unexamined Japanese Patent Publication No. 2003-35136 discloses a system wherein a delivery path and one portion of a communication path are formed by providing holes on a flange of an exhaust manifold. The delivery path is provided for guiding secondary air along a cylinder head. One end of the communication path is connected to the delivery path. Another end of the communication path is open toward an exhaust port.
However, in the first conventional system, the delivery pipe is supported by two communication pipes in a cantilever manner. Therefore, the delivery pipe is unstably fixed, and might be vibrated. Moreover, the bosses, for attaching the communication pipes, need to be provided with two ports connected to two branching pipes. As a result, there has been a problem that positions for disposing the bosses are limited, and a balancing measure cannot be easily provided when the amount of supplied air into respective branching pipes becomes unbalanced.
In the second conventional system, the delivery path is formed by providing a hole on the flange. However, since the length of the delivery path is long, making a hole for such a long path is difficult. Moreover, the communication path on the flange is made by providing a hole on the flange so that the communication path provided on the cylinder head can be connected thereto. However, depending on the position of the communication path of the cylinder head, in some cases the communication path cannot be formed on the flange. Therefore, there has been a problem that the position of the communication path of the flange cannot be flexibly set.
One of the purposes of the present invention is to provide a secondary air supply system for an internal combustion engine in which a measure to inhibit vibration can be easily provided, and the position of the supply path can be flexibly set. The secondary air supply system according to the present invention is preferably manufactured easily.